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2015 SOUTHEASTERN NATURALIST 14(1):66–84
Earthworm Communities in Previously Glaciated and
Unglaciated Eastern Deciduous Forests
Kristine N. Hopfensperger1,* and Sarah Hamilton2
Abstract - Native earthworms were removed from forested ecosystems during the last
glacial advance and have since been replaced with nonnative earthworm species. Nonnative
earthworms can cause major changes in microbial and plant communities and nutrient
cycling. In this study, we sought to compare the earthworm communities north and south
of the last glacial terminus, and to examine correlations between plant communities and
soil characteristics. In summer 2011, we measured the earthworm, herbaceous plant, and
woody plant communities in 3 forests in southwestern Ohio and 3 forests in northern Kentucky.
We also measured soil characteristics including moisture, pH, organic matter, and
nitrate and ammonium content. We found no native earthworm species at any of our study
sites; however, previously glaciated forests exhibited more diverse earthworm communities
and included all ecological groups. Earthworm species richness increased with increased
density of invasive woody plant species and decreased with increased soil ammonium.
Scientists and managers should continue to survey the earthworm communities in forests to
better understand the ranges of nonnative earthworms and the impacts they have on plant
communities and nutrient dynamics.
Introduction
In the US, exotic earthworms are dramatically altering nutrient cycling in forest
ecosystems (Bohlen et al. 2004a, c; Fisk et al. 2004; Groffman et al. 2004; Súarez et
al. 2004) and have been tied to microbial, plant, and animal community changes in
forests (Fisichelli et al. 2013; Frelich et al. 2006; McLean and Parkinson 2000a, b).
While studies of nonnative earthworms have become common in the northern hardwood
forests of the Northeast and Midwest in the past decades, earthworm surveys
and research have been lacking in the Southeastern Plains ecoregion, including areas
that span the last glacial terminus, such as southwestern Ohio and northern Kentucky.
The most recent glacial advance in North America (the Wisconsin glaciation;
12,000–25,000 y ago) removed native earthworms from areas north of the glacial
terminus (Gates 1970, Reynolds et al. 1974). Natural dispersal from unglaciated
areas into the northern regions has been slow (Terhivuo and Saura 2006). The introduction
of European and Asian earthworm species into these previously glaciated
areas started in the 1700s with European settlement (Gates 1966) and continues
today (Tiunov et al. 2006). Therefore, there is a mix of native and nonnative earthworm
species in areas south of the glacial limits (Reynolds 1970, Reynolds et al.
1974, Stebbings 1962), but northern, previously glaciated regions are dominated by
1Department of Biological Sciences, Northern Kentucky University, Highland Heights, KY
41099. 2Current address - Department of Forestry, University of Kentucky, Lexington, KY
40546. *Corresponding author - hopfenspek1@nku.edu.
Manuscript Editor: Lance Williams
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exotic earthworm species with few isolated observations of native species (Reynolds
et al. 2002).
An understanding of the dynamics between native and nonnative earthworm
species is beginning to take shape (Hendrix et al. 2006, Kalisz and Wood 1995).
Although data supporting resistance of native earthworms to invasion by nonnative
earthworm species is scarce (Hendrix et al. 2006), studies demonstrating co-occurrence
of native and nonnative earthworm species are more common (Abbott 1985,
James 1991, Stebbings 1962). However, it has not been determined if co-existence
is a persistent or transient state (Hendrix et al. 2006). Exotic earthworms are more
prevalent in areas that have been moderately to severely disturbed (Kalisz and
Dotson 1989, Kalisz and Wood 1995). Habitat disturbance may increase resource
availability, allowing nonnative earthworms to out-compete or co-exist with native
species (Fragoso et al. 1999, Winsome et al. 2006).
The presence of native and nonnative earthworms can have a dramatic effect
on soil-process characteristics, organic matter decomposition, soil structure, and
other biota (Bohlen et al. 2004b, Hale et al. 2006, Hendrix et al. 2006); therefore,
it is important to characterize the earthworm community to understand forestsoil
ecosystem processes. There is no current literature regarding earthworms in
southwestern Ohio; however, in 1928, Olson found no exotic Lumbricus terrestris
L. (Nightcrawler) or L. rubellus L. (Red Worm) in southwestern Ohio, but did
find exotic Allolobophora cholorticus Savigny and Aporrectodea turgida Eisen
(Mottled Worm) (Olson 1928). In a more recent study in southeastern Kentucky, the
exotic taxa Octolasion tyrtaeum Savigny, L. terrestris, L. rubellus, and L. castaneous
Savigny were found on relatively small and scattered disturbed sites (Kalisz
and Dotson 1989) in the more mountainous Appalachian Forest ecoregion. We are
uncertain if these findings are applicable to the entire region south of the glacial
terminus or restricted to the Appalachian Mountains.
Characterization of the earthworm community will provide insight into the
rates of forest ecosystem processes. For example, the results of many studies in
northern hardwood forests indicate that exotic earthworm species significantly
alter soil carbon, nitrogen, and phosphorus cycling (Bohlen et al. 2004a; Groffman
et al. 2004; Suárez et al. 2004, 2006a). Perhaps the most striking alteration
found in northern hardwood forests, is the potential of earthworms to transform
these forests from global carbon sinks into carbon sources (Bohlen et al. 2004b,
Lubbers et al. 2013). In the short-term, earthworm activity releases nutrients that
can increase nutrient-cycling rates (Bohlen et al. 2004b, Groffman et al. 2004)
leading to a net increase in carbon dioxide to the atmosphere (Fisk et al. 2004,
Li et al. 2003, Lubbers et al. 2013). However, in a meta-analysis, Lubbers et al.
(2013) reported that longer-term studies suggested initial carbon dioxide emissions
decreased with time leading to stabilization of organic carbon in the soil.
Native earthworm species may influence ecosystem processes for more of the
year than nonnative species because native earthworms are better adapted to local
climatic conditions than nonnative species (Callaham et al. 2001, James 1991).
In addition, Lachnicht et al. (2002) found reductions in carbon and nitrogen
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mineralization rates when native and nonnative species were in co-existence compared
to when the nonnative species was alone.
The nonnative earthworm species colonizing North America have highly invasive
characteristics (James and Hendrix 2004) and can cause remarkable changes in
soil structure and nutrient cycling depending on the ecological group to which they
belong. In general, earthworms prefer moist soil with neutral to basic soil pH (Curry
1998). Earthworms require calcium to supply their calciferous glands (Canti and
Pearce 2003), which produce calcium carbonate granules that moderate their blood
carbon dioxide levels and can increase soil pH when excreted (Crang et al. 1968).
Litter-dwelling epigeic species have minor impacts on soil structure and nutrient
concentrations by only mixing the O horizon of the soil (McLean and Parkinson
1997a, b). However, soil-dwelling endogeic species are known to mix surface litter
into the upper mineral soil horizons, thereby homogenizing the organic and mineral
layers and wholly removing the litter layer (Alban and Berry 1994, Langmaid 1964).
When species assemblages include both endogeic and anecic species which burrow
up to 2 m in depth, nutrient concentrations change the most, compared to soil without
anecic species present (Hale et al. 2005a). Native earthworm assemblages are generally
dominated by endogeic species (Fragoso et al. 1999, Kalisz 1993), which may
leave the soil surface open to invasion by epigeic exotic species.
Earthworm activity can impact forest plant communities in a variety of ways.
In field and mesocosm studies of a Pinus contorta Douglas ex Loudon (Lodgepole
Pine) forest, earthworms stimulated a shift from a fungal-dominated to bacterialdominated
soil (McLean and Parkinson 1998, 2000a, 2000b) causing the loss of
important mycorrhizal–plant root relationships (Wardle 2002). However, field studies
in a northern hardwood forest found that earthworm-induced elimination of the
O soil horizon led to an increase in bacteria over fungi (Dempsey et al. 2011, 2013).
Many native understory plants, such as Acer saccharum Marsh. (Sugar Maple),
are dependent on mycorrhizal relationships (Brundrett and Kendrick 1988), and
decrease in abundance when earthworms are present (Hale et al. 2006, Holdsworth
et al. 2007). In fact, Lawrence et al. (2003) found a decrease of mycorrhizal fungi
on Sugar Maple roots in earthworm-dominated plots. Earthworms can also alter
plant communities by consuming, and thereby reducing, the litter layer (Eisenhauer
et al. 2007, Gundale 2002, Hale et al. 2005a), which exposes plants to desiccation
and a more mineral-rich soil (Frelich et al. 2006, Heneghan et al. 2007). These
changes promoted by earthworms can result in reduced herbaceous-plant cover
(Hopfensperger et al. 2011) and a shift toward a more graminoid-dominated plant
community (Hale et al. 2006, Holdsworth et al. 2007, Nuzzo et al. 2009). Furthermore,
earthworms can directly affect plant communities through seed predation,
burial, and inducement or release of seed dormancy (Eisenhauer et al. 2009, Hopfensperger
et al. 2011, Regnier et al. 2008). As earthworms disturb native plant
communities, they may reduce competitive pressure thereby allowing invasive
plant species to dominate. Nuzzo et al. (2009) found that nonnative plant cover was
positively associated with earthworm biomass; however, there have been very few
studies relating earthworms to invasive-plant dynamics.
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We conducted surveys of the earthworm communities north (i.e., southwestern
Ohio) and south (i.e., northern Kentucky) of the last glacial terminus to characterize
current earthworm-community composition and to examine correlations between
earthworm and plant communities and between earthworm communities and
soil characteristics. We expected that: (1) earthworm communities in previously
glaciated forests would be dominated by nonnative species, but that native and nonnative
species would co-occur in earthworm communities of unglaciated forests;
(2) because unglaciated forests have a more diverse earthworm community with
increased ecosystem-process rates, these forests would have less soil organic matter
and a higher percent cover of invasive plant species; and (3) earthworm density
would increase with soil pH and with soil moisture.
Methods
Site description
We sampled in 3 previously glaciated forests in Hamilton and Butler counties,
OH (site 1 = Richardson Forest Preserve, site 2 = Winton Woods Park, site 3 =
Miami Whitewater Forest) and 3 previously unglaciated forests in Campbell and
Kenton counties, KY (site 4 = Hawthorne Crossing Conservation Area, site 5 = AJ
Jolly Park, site 6 = Morning View Heritage Land) for a total of 6 sampled forests
(Fig. 1 includes geographic coordinates). Both previously glaciated and unglaciated
forests lie near the terminus of the last glacial advance (Ray 1974). The glaciated
sites are included in the Till Plains section of the Central Lowland Physiographic
Province (Fenneman 1916). The previously unglaciated sites are in the Outer Bluegrass
Region of the Interior Low Plateau (Brockman 1998). The soils in all study
forests are silt loam or silty clay loam and are classified as mesic Typic Hapludalfs
(USDA NRCS http://websoilsurvey.nrcs.usda.gov). Climate in the study region is a
continental type with cold winters (average January high temperature = -1 °C), hot
summers (average July high temperature = 27 °C), and average annual precipitation
of ~112 cm (NOAA 2013). Forests of the region have been thoroughly described
by Dr. E. Lucy Braun and many others (Braun 1916, 1936, 1950; Bryant 1987,
2004; Kuchler 1964). The area is typical of the mixed mesophytic forest region
characterized by dominant species including Fagus grandifolia Ehrh. (American
Beech), Fraxinus americana L. (White Ash), Sugar maple, Quercus rubra L. (Red
Oak), and Prunus serotina Ehrh. (Black Cherry). None of the forests studied are
considered old growth and all of them have had some history of harvesting in the
past (K.N. Hopfenperger, pers. observ.).
Plant-community sampling
We delinated three 400-m2 stands in each of the 6 forest sites and randomly
placed one 5-m2 plot within each quadrant of each stand for a total of 12 plots per
forest and 72 project plots. We chose stands with tree-canopy species and cover
to minimize canopy effect on the earthworm communities and measured soil variables.
All stands were dominated by Sugar Maple with a mix of other species,
including Fraxinus (ash) and Aesculus (buckeye). We identified all trees within
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each plot and measured their diameter at breast height (1.4 m; DBH) and also
identified all saplings, seedlings, and shrubs within each plot. We recorded percent
cover (modified methods of Braun-Blanquet 1964) of all herbaceous species within
a 0.75-m radius around the center of the plot. We conducted our sampling in July
because we thought that would be the time of year when the plant community was at
peak biomass. Plant species richness, diversity—using the Shannon diversity index
(Shannon and Weaver 1949),—and density of invasive herbaceous plant and woody
species were calculated for each plot.
Earthworm sampling
We sampled earthworm communities in May 2011, when we felt conditions
would be optimal for earthworm movement and sampling due to moisture and
temperature conditions then. We designated a 30 cm by 30 cm subplot in the center
of each plot, and carefully removed litter from the sample area. We slowly poured
a solution of 4 L of water mixed with 40 g of ground yellow mustard seed over
the plot to stimulate movement of earthworms to the soil surface for collection
Figure 1. Map of 6 study sites sampled for earthworm and plant communities and soil
characteristics in 2011. Three sites in Ohio were previously glaciated forests, and 3 sites in
Kentucky were unglaciated forests.
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(Lawrence and Bowers 2002); earthworms were collected from each plot for 15
minutes. Upon collection, we rinsed each earthworm with water and placed it in
70% isopropyl alcohol for transportation back to the lab. Within 24 hours of being
extracted from the field, the earthworms were placed in formalin to fix their tissues.
After fixation in formalin, we placed earthworms back into 70% isopropyl alcohol
for long-term storage.
We identified adult earthworms to species, categorized immature earthworms as
either “Lumbricus immature” or “other immature”, and calculated percent of immature
earthworms for each plot. Earthworms were separated by species per plot
and dried at 60 °C for 48 h. We determined ash-free dry biomass (AFDM) by ashing
the worms in a muffle furnace at 500 °C for 4 h. For each plot, we calculated
earthworm species richness, diversity, and density (including numbers of immature
earthworms), as well as the percent immature.
Soil sampling
We collected 3 replicate soil cores (2.54 cm diameter x 10 cm depth) adjacent
to each subplot on the same day as we extracted worms. Soil temperature was
recorded in the field for each plot during worm and soil-core extraction. Samples
were stored and transported on ice in the field and then stored in a cooler at 4 °C
until processing. We homogenized soil cores from each plot and passed the samples
through a 2-mm-mesh sieve to remove large roots and rocks. We measured the
pH of 2 subsamples from each plot and averaged them following the protocol of
Robertson et al. (1999). Soil samples were then dried at 70 °C to a constant mass
to obtain gravimetric water content (i.e., soil moisture; Jarrell et al. 1999). Soil
organic matter content was obtained using the loss-on-ignition technique (Nelson
and Sommers 1996).
Soil nitrate-N (NO3
--N) and ammonium-N (NH4
+-N) were extracted from each
sample with 2M KCl. We measured NH4
+-N colorimetrically using a microplate
reader following a method that replaces phenol with the sodium salt of 2-phenylphenol
(PPS) as the substrate for the Berthelot reaction (Rhine et al. 1998, Sims
2006, Sims et al. 1995). We also measured NO3
--N colorimetrically on a microplate
reader using an enzyme method to convert NO3
- to NO2
- (Campbell et al. 2006,
Ringuet et al. 2011). In this process, AtNaR2 (acquired from NECi, Lake Linden,
MI) quantitatively reduces nitrate to nitrite in a phosphate buffer. All nitrite then
diazotizes with sulfanilamide and then reacts with N-(1-Napthyl)ethylenediamine
to form a pink color absorption that is read at 540 nm (Patton and Kryskalla 2011).
Statistical analyses
To test for differences between previously glaciated and unglaciated forests,
we used nested mixed-model analysis of variance. Results from the 3 stands (12
plots total) within each of the 6 forest sites were averaged and nested within the 2
treatments (glaciated vs. unglaciated forests). In the mixed models, random effects
were all the possible forests within the treatments, and the main effect (independent
variable) was treatment. We ran nested mixed-models for all measured (dependent)
variables (i.e., plant-community metrics, earthworm-community metrics, and
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soil characteristics). Using Pearson correlation analysis, we tested relationships
between earthworm metrics and the plant community and soil variables. We chose
correlation analysis over regression because the study was not designed to test
cause and effect and there are plausible reasons for both earthworms affecting soil
and plant variables and vice versa. Correlation analyses were performed on the average
of all replicate plots within each forest for each measured variable (n = 6). We
checked all measured variables to verify that they met the assumption of normality.
Statistical analyses were conducted in SAS system for Windows (SAS Institute v8).
Results
Differences between glaciated and unglaciated forests
We did not find distinct differences between the earthworm communities of previously
glaciated and unglaciated forests. Earthworm communities in previously
glaciated forests had slightly greater earthworm biomass than sampled plots from
unglaciated forests (F = 5.81, P = 0.07); however, earthworm density, species richness,
and diversity did not differ between the treatments (Table 1). We found 11
different earthworm species from all sampled locations; none were native (Table 2).
Four earthworm species were of the epigeic ecological group, 6 species were endogeic,
and 1 species sampled, L. terrestris, was anecic. Earthworm species with
the greatest densities found during the study included L. rubellus, L. terrestris, and
Aporrectodea rosea Savigny; however, L. terrestris was more commonly found in
the previously glaciated forests, while L. rubellus was more commonly found in the
unglaciated forests (Fig. 2).
Species richness and diversity of the herbaceous and woody plant communities
of glaciated and unglaciated forests did not differ from each other (Table 1). Overall,
we found a total of 58 plant species in the herbaceous plant-cover survey of all
sites. The 2 dominant herbaceous species were Allaria petiolata (M. Bieb.) (Garlic
Table 1. Nested mixed-model results given as mean ± standard error for measured variables in previously
glaciated and unglaciated forest plots. df1, df2 = 1, 4.
Dependent variable Glaciated Unglaciated F-statistic P-value
Earthworm biomass (AFDM m-2) 11.90 ± 2.28 5.17 ± 1.21 5.81 0.07
Earthworm density (worms m-2) 184.00 ± 33.20 140.00 ± 36.70 0.26 0.64
Earthworm species richness 23.90 ± 0.34 1.64 ± 0.24 1.63 0.27
Earthworm diversity 0.73 ± 0.13 0.38 ± 0.09 3.49 0.14
Percent immature earthworms 63.40 ± 8.61 75.40 ± 6.35 0.47 0.53
Herbaceous species richness 5.56 ± 0.49 6.00 ± 0.50 0.17 0.70
Woody species richness 5.42 ± 0.57 3.56 ± 0.41 2.87 0.17
Total vegetative cover 17.90 ± 1.68 19.90 ± 3.14 0.10 0.77
# herbaceous invasive species 1.33 ± 0.24 1.08 ± 0.18 0.21 0.67
# woody invasive species 0.78 ± 0.18 0.61 ± 0.16 0.25 0.64
Soil pH 6.02 ± 0.07 6.33 ± 0.12 1.57 0.28
Soil temperature (°C) 19.50 ± 0.58 18.80 ± 0.57 0.97 0.38
Soil moisture (in situ, %) 57.00 ± 4.92 53.20 ± 4.35 0.12 0.75
Soil nitrate (mg N kg-1) 2.16 ± 1.05 6.05 ± 1.38 2.30 0.20
Soil ammonium (mg N kg-1) 9.01 ± 0.94 11.80 ± 0.92 2.43 0.19
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Mustard) and Potentilla simplex Michx. (Common Cinquefoil). Garlic mustard was
the only herbaceous species found at every study site. We recorded a total of 34
woody species among all surveyed forests; Sugar Maple was dominant by threefold,
followed by Fraxinus pennsylvanica Marshall (Green Ash) and Acer negundo
L. (Boxelder). Less-common woody species included White Ash, Lonicera maackii
(Rupr.) Herder (Amur Honeysuckle), and Black Cherry. Sugar Maple and Amur
Honeysuckle were the only woody species found at every study site. We recorded
in the plant surveys 5 species commonly thought of as invasive including Garlic
Mustard, Euonymus fortunei (Turcz.) Hand.-Maz. (Wintercreeper), Lysimachia
nummularia L. (Moneywort), Rosa multiflora Thunb. (Multiflora Rose), and Amur
Honeysuckle (Table 3). We recorded 5 invasive plant species in 2 of the previously
glaciated forests and 3–4 in the unglaciated forests (Fig. 3). Soil characteristics
Table 2. Ecological group and average density (worms m-2) of all earthworm species collected during
sampling at each forest site (n = 12).
Glaciated Unglaciated
Ecological
Earthworm species group Site 1 Site 2 Site 3 Site 4 Site 5 Site 6
Aporrectodea rosea Epigeic 2.7 17.7 0.0 0.0 1.4 13.6
Dendrobaena octaedra Epigeic 2.0 0.7 0.0 0.0 0.0 0.0
Dendrodrilus rubidus Epigeic 0.0 0.0 0.7 1.4 0.0 0.0
Lumbricus castaneus Epigeic 3.4 0.7 0.0 0.0 0.0 0.7
Allobophora chlorotica Endogeic 0.0 12.9 7.5 3.4 4.1 0.0
Aporrectodea caliginosa Endogeic 0.0 1.4 0.0 0.0 0.7 0.0
Aporrectodea trapezoides Endogeic 0.0 5.4 2.7 0.0 2.0 1.4
Aporrectodea turgida Endogeic 17.0 2.7 2.0 0.7 0.0 0.0
Lumbricus rubellus Endogeic 0.7 6.8 8.2 0.0 10.2 27.9
Octolasion tyrtaeum Endogeic 0.0 2.0 4.8 0.0 2.0 6.8
Lumbricus terrestris Anecic 21.8 10.2 0.7 1.4 1.4 0.0
Lumbricus immature 46.9 71.4 207.5 0.0 101.4 209.5
Other immature 12.2 34.0 47.6 10.2 12.2 12.9
Figure 2. Average
density of worms
m-2 at each sampled
forest site for
the 3 most-dominant
earthworm
species collected
during the study.
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including soil temperature, moisture, pH, and inorganic nitrogen did not vary between
previously glaciated and unglaciated forests (Table 1).
Interactions between earthworms and their environment
We found many significant correlations between earthworms and their physical,
chemical, and biological environment (Table 4). Earthworm species richness
and diversity were positively correlated with abundance of invasive woody species
(Amur Honeysuckle and Multiflora Rose) (P < 0.05; Table 4). In addition, the
percent of immature earthworms was negatively correlated with the abundance of
herbaceous invasive species (P < 0.05; Table 4). Earthworm density and percent
of immature earthworms were negatively correlated with soil moisture (P < 0.03;
Fig. 4). Percent of immature earthworms was also negatively correlated with
soil temperature (P < 0.01; Table 4). Earthworm diversity was negatively correlated
with soil ammonium (P < 0.05; Fig. 4). We found no significant correlations
between any earthworm metrics and soil pH or soil nitrate level.
Table 3. Average herbaceous percent cover and total number of woody stems for all invasive plant
species found during peak growth 2011. Twelve 5-m2 plots were sampled per site. Average % cover
per plot given for herbaceous species, and total # of stems given for woody species.
Glaciated Unglaciated
Species Site 1 Site 2 Site 3 Site 4 Site 5 Site 6
Invasive herbaceous species
Allaria petiolata 4.54 1.22 0.75 6.0 0.08 3.08
Euonymus fortunei 0.04 1.04 0.00 0.00 0.00 0.00
Lonicera maackii 4.08 0.46 0.00 0.00 0.17 0.00
Rosa multiflora 0.04 0.00 0.00 0.04 0.00 0.00
Lysimachia nummularia 0.00 0.00 0.00 28.4 0.00 0.04
Invasive woody species
Lonicera maackii 55 50 11 0 23 22
Rosa multiflora 0 10 1 0 0 16
Figure 3. Invasive
plant species richness
for each sampled
forest site.
A total of 7 invasive
herbaceous
and woody species
were found
throughout the
project sites.
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Discussion
Earthworm density and richness did not differ between previously glaciated and
unglaciated forests, and most interestingly, we found only nonnative earthworm
species at all sampled sites. Earthworm communities at our sites were more similar
to those found in the northern hardwood, glaciated forests of Minnesota (Hale et
al. 2005b, Reynolds et al. 2002) and New York (Stoscheck et al. 2012, Suarez et al.
2006b), where no native species were found, than to those found in the southeastern
US (Hendrix et al. 1992, Kalisz and Dotson 1989). Native earthworm species
including Sparganophilus eiseni Smith and Diplocardia singularis Ude were once
found in the region (Olson 1928); therefore, our current data suggests that native
species have been displaced by exotics like Lumbricus rubellus. Others suggest that
Table 4. Significant correlations between measured earthworm variables with plant community variables
and soil characteristics.
Correlated metric Earthworm metric r value P-value
Earthworm correlations with plant variables
# invasive herbaceous species Percent immature -0.83 0.04
# invasive woody species Earthworm species richness 0.87 0.02
# invasive woody species Earthworm diversity 0.82 0.04
Earthworm correlations with soil variables
Soil ammonium Earthworm diversity -0.83 0.04
Soil moisture Earthworm density -0.85 0.03
Soil moisture Percent immature -0.93 0.01
Soil temperature Percent immature -0.96 less than 0.01
Figure 4. Earthworm density and percent of immature earthworms (measured per m2 and
averaged per site) decreased with soil moisture, percent of immature earthworms decreased
with soil temperature, and earthworm diversity (measured using the Shannon index) decreased
with soil ammonium among the study sites.
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the same phenomenon of exotic earthworm species replacing natives has occurred
in the southeastern US (Kalisz and Dotson 1989, Reynolds 1972). Site variability
may have masked differences in earthworm communities that might have been apparent
if the soils and vegetation at our sites had been more similar. For example,
we found differences in dominant earthworm species, dominant invasive plant
species, and soil parameters among forest sites within the previously glaciated and
unglaciated treatments.
Although earthworm communities found in both the previously glaciated and
unglaciated forests contained all three ecological earthworm groups (i.e., epigeic,
endogeic, and anecic), the communities in previously glaciated sites had higher
densities of anecic earthworms, and the unglaciated forests had higher densities of
endogeic earthworms (Fig. 2). High densities of endogeic earthworms are characteristic
of soils dominated by native earthworms (Fragoso et al. 1999, Kalisz 1993).
We predicted, but did not find to be true, that forests on unglaciated sites would
have a more diverse earthworm community with a mixture of native and nonnative
species occupying more niche spaces than in the forests at previously glaciated
sites. Earthworm communities that occupy multiple niches have a greater impact
on forests by changing soil structure and altering soil nutrients with the relocation
of organic matter in multiple soil layers (Bohlen et al. 2004c, Frelich et al. 2006,
Hale et al. 2005a, Hopfensperger et al. 2011). When a multi-species earthworm
community removes soil organic matter from the surface, the loss of available
nutrients is magnified compared to single-habit earthworm invasions (Hale et al.
2005a, Sackett et al. 2012, Suarez et al. 2004). In fact, we found that forests in both
previously glaciated and unglaciated areas with all ecological earthworm groups
represented had lower soil-N concentrations (Fig. 4C). In addition to impacting
soil dynamics, earthworm communities containing all ecological groups have also
been found to have a greater effect on plant communities than sites with less diverse
earthworm communities (Hale et al. 2006, Hopfensperger et al. 2011). However,
Hale et al. (2006) found that the presence of a specific species, Lumbricus rubellus,
was most closely tied to sites where the plant community was dominated by
invasive and nonnative plant species. We found higher densities of L. rubellus in 2
of the 3 unglaciated forest plots; however, these plots did not have greater invasive
plant density.
When we combined data from previously glaciated and unglaciated forests,
we found that earthworm species richness and diversity increased with number of
invasive woody plant species. Though we did not find our prediction of a co-occurrence
of native and nonnative species resulting in greater earthworm diversity
in unglaciated forests to be true, we predicted and found evidence of an increase
of invasive plants with increased earthworm diversity; however, we obtained this
result only for our previously glaciated sites. Perhaps as others have suggested,
increased disturbance allowed for greater diversity of nonnative earthworms and
higher densities of invasive plants at these sites (Kalisz and Dotson 1989, Kalisz
and Wood 1995). Nuzzo et al. (2009) found that earthworm biomass increased
with greater nonnative plant cover, which they attributed to earthworms altering
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2015 Vol. 14, No. 1
soil nutrients and/or disrupting plant mycorrhizae, creating an ideal environment
for nonnative plants to dominate. Another way earthworms may facilitate
invasion of nonnative plants is through direct removal of the O horizon and soil
organic matter, leading to a decline in native herbaceous cover (Alban and Berry
1994; Bohlen et al. 2004a, c; Hale et al. 2004, 2005b), which could open-up the
forest floor to colonization by new and/or invasive species. Indeed, we observed
that previously glaciated sites with high earthworm diversity also had high densities
of invasive Garlic Mustard, Amur Honeysuckle, and Multiflora Rose, which
exploit disturbed areas (Doll 2006, Luken 1988, Szafoni 1991). In addition, these
latter 2 are woody invasive species that bear fruit eaten and readily dispersed by
wildlife and that commonly become established in forests with sparse ground
cover and a thin O horizon (Baskin and Baskin 1998, Grime 1979).
In contrast to the positive correlation between woody invasive species and earthworm
diversity and richness, we found that the percent of immature earthworms in
the earthworm community was negatively correlated with invasive herbaceous plant
cover. The dominant invasive herbaceous species at our sites was Garlic Mustard,
which is known to exhibit allelopathic properties (Prati and Bossdorf 2004). Plants
that contain herbivore-repellant secondary compounds may also be avoided by earthworms.
For example, Hale et al. (2006) found that earthworms avoided consuming
toxic compounds produced by Arisaema triphyllum (L.) Schott (Jack-in-the-Pulpit)
and Allium tricoccum Aiton (Wild Leek). Currently, there is no published data linking
allelopathy from Garlic Mustard to earthworms; however, Garlic Mustard is known
to contain compounds that inhibit feeding by butterfly larvae (Haribal et al. 2001).
Perhaps immature earthworms at our study sites avoid occupying areas that have a
high percent cover of Garlic Mustard due to the same compounds that inhibit butterfly
larvae. Indeed, we found the lowest earthworm density at site 4 (Fig. 2), which had
the greatest percent cover of Garlic Mustard (Table 3). In addition, the dominant species
found at site 4 was L. terrestris, an anecic species whose deep burrows may help
them to avoid the allelopathic effects of Garlic Mustard. Another native but invasive
species known to increase with earthworm biomass, Carex pensylvanica Lam. (Pennsylvania
Sedge), is known to invade forest floors (Aikens et al. 2007, Hopfensperger
et al. 2011). Pennsylvania Sedge responds to the presence of endogeic earthworm
species such as L. rubellus by creating new root systems via basal meristems and
spreading vegetatively through the developing A horizon (Hale et al. 2006). Pennsylvania
Sedge was a dominant understory species at sites 3 and 5, which were also
dominated by endogeic earthworm species (Fig. 2).
We found aspects of earthworm communities from both previously glaciated and
unglaciated forests correlated with soil moisture, temperature, and soil ammonium
at our study sites. We observed high earthworm diversity in areas with low soil
ammonium (Fig. 4C), which may indicate that diverse earthworm communities
occupying multiple soil niches may increase soil-N cycling. Results of published
studies concerning earthworms and soil N dynamics are mixed. For example, accelerated
N-process rates in the presence of nonnative earthworms may both increase
and decrease inorganic N concentrations (Bohlen et al. 2004c). Sackett et al. (2012)
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2015 Vol. 14, No. 1
78
found that soil NH4
+-N increased with soil pH and offset the amount of N lost from
increased decomposition and organic matter removal by earthworms. We predicted
an increase in soil pH with earthworm density; however, we did not find any correlations
between earthworm community metrics and soil pH. The absence of a clear
relationship between earthworms and soil pH in our study is noteworthy because
many others have found that earthworm density increases with soil pH (Burtelow
et al. 1998, Fisichelli et al. 2013, Hopfensperger et al. 2011); acidic soil inhibits
many earthworm species because of their need for calcium (Canti and Pearce 2003,
Curry 2004). Perhaps soil pH was not variable enough among our sites to determine
a trend in the data (Table 1). Lastly, at our study sites, earthworm density was
highest at moderate levels of soil moisture (between 25% and 60%), but decreased
at higher moisture levels, which is counter to what we predicted. We recorded no
earthworms when soil moisture was below 25% (Fig. 4). This observation suggests
that there is a threshold beyond which soil can become too moist for earthworms,
resulting in less than optimal conditions with lower earthworm densities.
Previously glaciated forests with earthworm communities containing all 3 ecological
groups had lower soil pH and lower N concentrations than the unglaciated
forests that lacked anecic earthworms. When anecic earthworms transport fresh
organic litter from the forest floor to the A horizon, they may have a great impact on
soil-nutrient dynamics (Sackett et al. 2012; Suarez et al. 2006a, b). We conducted
our soil pH and nutrient-content measurements on samples taken in the top 20 cm
of the soil; anecic earthworms relocate fresh litter to well below that soil depth.
The removal of the fresh litter from the surface relocates the ecosystem’s nutrient
source to a depth below which our soil samples were collected, which may explain
the lower inorganic N concentrations recorded in the surface soils (Bohlen et al.
2004a, Sackett et al. 2012). N loss may also occur by leaching from the soil surface
down the flowpaths of anecic earthworm burrows (Suarez et al. 20 04, Subler et al.
1997). In addition, the movement of organic matter away from the surface by anecic
earthworm species results in much greater mixing of the soil compared to soil
found in areas without anecic earthworms (Bohlen et al. 2004a, Suarez et al. 2004).
Greater soil mixing in the presence of anecic species may prevent the maintenance
of a higher soil pH than would occur in earthworm communities without anecic
earthworms (Burtelow et al. 1998, Hopfensperger et al. 2011). Perhaps the prevalence
of anecic earthworms at our previously glaciated study sites was due to past
land use. Non-native earthworm establishment is related to degree of disturbance
and human activity; future studies could investigate and quantify the difference in
the degree of disturbance between the previously glaciated and unglaciated sites.
The 3 previously glaciated forests we studied are in protected forest preserves, 1
unglaciated forest is in a protected forest preserve and the other 2 unglaciated sites
are recently protected forests that were once farmsteads.
Summary
We studied earthworm communities in forests directly north and south of the last
glacial terminus in a region of the US that had not been surveyed for earthworms in
almost 100 years. We did not observe native earthworm species during our study,
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K.N. Hopfensperger and S. Hamilton
2015 Vol. 14, No. 1
and earthworm density and species richness did not differ between previously glaciated
and unglaciated forests, perhaps due to the variability among the study sites
within the treatments. However, previously glaciated forests contained earthworm
communities with species occupying more niche spaces than we observed in unglaciated
forests. Specifically, the high density of anecic earthworms we recorded
in previously glaciated forests may be an artifact of site disturbance and human
activity. The presence of anecic earthworms in the community was associated with
and may have generated lower soil pH and lower soil N concentrations. It is possible
that the effect of anecic earthworms on the soil created conditions conducive
for the establishment of invasive plant species. Due to the major impacts nonnative
earthworms can have on the plant communities and nutrient dynamics of our forest
ecosystems, scientists and managers should continue to survey the earthworm
and plant communities throughout the US to better understand the movement of
native and exotic earthworm species. Our results suggest that nonnative earthworm
species have been displacing native earthworm species in southwestern Ohio and
northern Kentucky forests over the last 100 years, providing support that co-occurrence
of native and nonnative earthworm species may be a transient phenomenon.
Acknowledgments
Funding was provided by the Northern Kentucky University College of Arts and Sciences
and the Project FORCE UR-STEM Program, Grant No. DUE-STEP-096928. We extend
special thanks to Mary Kathryn Dickerson from the Campbell and Kenton counties Soil and
Water Conservation District, AJ Jolly Parks, and Hamilton County Parks for access to study
sites. We also thank research student Sarah Haley Shaw for her help wit h field sampling.
Literature Cited
Abbott, I. 1985. Distribution of introduced earthworms in northern Jarrah forest of Western
Australia. Australian Journal of Soil Resources 23:263–270.
Aikens, M.L., D. Ellum, J.J. McKenna, M.J. Kelty, and M.S. Ashton. 2007. The effects of
disturbance intensity on temporal and spatial patterns of herb colonization in a southern
New England mixed-oak forest. Forest Ecology and Management 252:144–158.
Alban, D.H., and E.C. Berry. 1994. Effects of earthworm invasion on morphology, carbon,
and nitrogen of a forest soil. Applied Soil Ecology 1:243–249.
Baskin, C.C., and J.M. Baskin. 1998. Seeds: Ecology, Biogeography, and Evolution of Dormancy
and Germination. Academic Press, San Diego, CA. 666 pp.
Bohlen, P.J., P.M. Groffman, T.J. Fahey, and M. Fisk. 2004a. Influence of earthworm invasion
on redistribution and retention of soil carbon and nitrogen in northern temperate
forests. Ecosystems 7:13–27.
Bohlen, P.J., S. Scheu, C.M. Hale, M.A. McLean, S. Migge, P.M. Groffman, and D. Parkinson.
2004b. Invasive earthworms as agents of change in north temperate forests.
Frontiers in Ecology and the Environment 8:427–435.
Bohlen, P.J., D.M. Pelletier, P.M. Groffman, T.J. Fahey, and M.C. Fisk. 2004c. Influence of
earthworm invasion on redistribution and retention of soil carbon and nitrogen in northern
temperate forests. Ecosystems 7:13–27.
Braun, E.L. 1916. The physiographic ecology of the Cincinnati region. Ohio Biological
Survey 2:113–211.
Southeastern Naturalist
K.N. Hopfensperger and S. Hamilton
2015 Vol. 14, No. 1
80
Braun, E.L. 1936. Forests of the Illinoian Till Plain in southwestern Ohio. Ecological
Monographs 6:90–149.
Braun, E.L. 1950. Deciduous Forests of Eastern North America. Blakiston Company, Philadelphia,
PA. 596 pp.
Braun-Blanquet, J. 1964. Pflanzensoziologie. Springer-Verlag, Berlin, Germany. 631 pp.
Brockman, C.S. 1998. Physiographic Regions of Ohio [map]. Division of Geological Survey,
State of Ohio, Columbus, OH.
Brundrett, M.C., and B. Kendrick. 1988. The mycorrhizal status, root anatomy, and phenology
of plants in a Sugar Maple forest. Canadian Journal of Botany 66:1153–1173.
Bryant, W.S. 1987. Structure and composition of the old-growth forests of Hamilton County,
Ohio and environs. Pp. 317–324, In R.L. Hay, F.W. Woods, and H. DeSelm (Eds.).
Proceedings of the 6th Central Hardwoods Forest Conference. Nashville, TN. 526 pp.
Bryant, W.S., and M.E. Held. 2004. Forest vegetation in Hamilton County, Ohio: A cluster
analysis and ordination study. Pp. 312–321, In D.A. Yaussy, D.M. Hix, R.P. Long, and
P.C. Goebel (Eds.). Proceedings of the 14th Central Hardwood Forest Conference. Newtown
Square, PA. 553 pp.
Burtelow, A.E., P.J. Bohlen, and P.M. Groffman. 1998. Influence of exotic earthworm invasion
on soil organic matter, microbial biomass, and denitrification potential in forest
soils of the northeastern United States. Applied Soil Ecology 9:197–202.
Callaham, M.A., Jr., J.M. Blair, and P.F. Hendrix. 2001. Native North American and introduced
European earthworms in tallgrass prairie: Behavioral patterns and influences on
plant growth. Biology and Fertility of Soil 34:49–56.
Campbell, W.H., P. Song, and G.G. Barbier. 2006. Nitrate reductase for nitrate analysis in
water. Environmental Chemistry Letters 4:69.
Canti, M.G., and T.G. Pearce. 2003. Morphology and dynamics of calcium carbonate granules
produced by different earthworm species. Pedobiologia 47:511–521.
Crang, R.E., R.C. Holsen, and J.B. Hitt. 1968. Calcite production in mitochondria of earthworm
calciferous glands. Bioscience 18:299–301.
Curry, J.P. 1998. Factors affecting earthworm abundance in soils. Pp. 37–64, In A. Edwards
(Ed.). Earthworm Ecology. St.Lucie Press, Boca Raton, FL. 448 pp.
Dempsey, M.A., M.C. Fisk, and T.J. Fahey. 2011. Earthworms increase the ratio of bacteria
to fungi in northern hardwood forest soils, primarily by eliminating the organic horizon.
Soil Biology and Biochemistry 43:2135–2141.
Dempsey, M.A., M.C. Fisk, J.B. Yavitt, T.J. Fahey, and T.C. Balser. 2013. Exotic earthworms
alter soil microbial community composition and function. Soil Biology and
Biochemistry 67:263–270.
Doll, J.D. 2006. Biology of Multiflora Rose. North Central Weed Science Society Proceedings
61:239.
Eisenhauer, N., S. Partsch, D. Parkinson, and S. Scheu. 2007. Invasion of a deciduous forest
by earthworms: Changes in soil chemistry, microflora, microarthropods, and vegetation.
Soil Biology and Biochemistry 39:1099–110.
Eisenhauer, N., M. Schuy, O. Butenschoen, and S. Scheu. 2009. Direct and indirect effects
of endogeic earthworms on plant seeds. Pedobiologia 52:151–162.
Fenneman, N.M. 1916. The geology of Cincinnati and vicinity. Ohio Geological Survey
Bulletin 9. Columbus, OH.
Fisichelli, N.A., L.E. Frelich, P.B. Reich, and N. Eisenhauer. 2013. Linking direct and indirect
pathways mediating earthworms, deer, and understory composition in Great Lakes
forests. Biological Invasions 15:1057–1066.
Fisk, M.C., T.J. Fahey, P.M. Groffman, and P.J. Bohlen. 2004. Earthworm invasion, fineroot
distributions, and soil respiration in north temperate forests. Ecosystems 7:55–62.
Southeastern Naturalist
81
K.N. Hopfensperger and S. Hamilton
2015 Vol. 14, No. 1
Fragoso, C., P. Lavelle, E. Blanchart, B. Senapati, J. Jimenez, M. de los Angeles Martinez,
T. Decaens, and J. Tondoh. 1999. Earthworm communities of tropical agroecosystems:
Origin, structure, and influences of management practices. Pp. 27–55, In P. Lavelle, L.
Brussaard, and P. Hendrix (Eds.). Earthworm Management in Tropical Agroecosystems.
CABI Publishing, New York, NY. 320 pp.
Frelich, L., C. Hale, S. Scheu, A. Holdsworth, L. Heneghan, P. Bohlen, and P. Reich. 2006.
Earthworm invasion into previously earthworm-free temperate and boreal forests. Biological
Invasions 8:1235–45.
Gates, G.E. 1966. Requiem for megadrile utopias: A contribution toward the understanding
of the earthworm fauna of North America. Proceedings of the Biological Society of
Washington 79:239–254.
Gates, G.E. 1970. Miscellanea megadrilogica VIII. Megadrilogica 1:1–14.
Grime, J.P. 1979. Plant Strategies and Vegetation Processes. John Wiley and Sons, Chichester,
UK. 456 pp.
Groffman, P.M., P.J. Bohlen, M.C. Fisk, and T.J. Fahey. 2004. Exotic earthworm invasion
and microbial biomass in temperate forest soils. Ecosystems 7:45–54.
Gundale, J.M. 2002. Influence of exotic earthworms on the soil organic horizon and the rare
fern Botrychium mormo. Conservation Biology 16:1555–1561.
Hale, C.M., L.E. Frelich, P.B. Reich, and J. Pastor. 2005a. Effects of European earthworm
invasion on soil characteristics in northern hardwood forests of Minnesota, USA. Ecosystems
8:911–927.
Hale, C.M., L.E. Frelich, and P.B. Reich. 2005b. Exotic European earthworm invasion
dynamics in northern hardwood forests of Minnesota, USA Ecological Applications
15:848–860.
Hale, C.M., L.E. Frelich, and P.B. Reich. 2006. Changes in hardwood forest understory-plant
communities in response to European earthworm invasions. Ecology 87:1637–1649.
Haribal, M., Z. Yang, A.B. Attygalle, J.A.A. Renwick, and J. Meinwald. 2001. A cyanoallyl
glucoside from Alliaria petiolata, as a feeding deterrent for larvae of Pieris napi oleracea.
Journal of Natural Products 64:440–443.
Hendrix, P.F., B.R. Mueller, R.R. Bruce, G.W. Langdale, and R.W. Parmelee. 1992. Abundance
and distribution of earthworms in relation to landscape factors on the Georgia
Piedmont, USA Soil Biology and Biochemistry 24:1357–1361.
Hendrix, P.F., G.H. Baker, M.A. Callaham, Jr., G.A. Damoff, C. Fragoso, G. González, S.W.
James, S.L. Lachnicht, T. Winsome, and X. Zou. 2006. Invasion of exotic earthworms
into ecosystems inhabited by native earthworms. Biological Invasions 8:1287–300.
Heneghan, L., J. Steffen, and K. Fagen. 2007. Interactions of an introduced shrub and introduced
earthworms in an Illinois urban woodland: Impact on leaf-litter decomposition.
Pedobiologia 50:543–551.
Holdsworth, A.R., L.E. Frelich, and P.B. Reich. 2007. Effects of earthworm invasion on
plant species richness in northern hardwood forests. Conservation Biology 21:997–1008.
Hopfenpserger, K.N., G.M. Leighton, and T.J. Fahey. 2011. Influence of invasive earthworms
on above and belowground vegetation in a northern hardwood forest. American
Midland Naturalist 166:53–62.
James, S.W. 1991. Soil, nitrogen, phosphorus, and organic matter processing by earthworms
in tallgrass prairie. Ecology 72:2101–2109.
James, S.W., and P.F. Hendrix. 2004. Invasion of exotic earthworms into North America and
other regions. Pp. 75–88, In C.A. Edwards (Ed). Earthworm Ecology, 2nd Edition. CRC
Press, Boca Raton, FL. 456 pp.
Southeastern Naturalist
K.N. Hopfensperger and S. Hamilton
2015 Vol. 14, No. 1
82
Jarrell, W.M., D.E. Armstrong, D.F. Grigal, E.F. Kelley, H.C. Monger, and D.A. Wedin.
1999. Soil water and temperature status. Pp. 55–73, In G.P. Robertson, D.C. Coleman,
C.S. Bledsoe, and P. Sollins (Eds.). Standard Soil Methods for Long-term Ecological
Research. Oxford University Press, New York, NY. 462 pp.
Kalisz, P. 1993. Native and exotic earthworms in deciduous forest soils of eastern North
America. Pp. 93–100, In B.N. Knight (Ed.). Biological Pollution: The Control and Impact
of Invasive Exotic Species. Indiana Academy of Science, Indianapolis, IN. 270 pp.
Kalisz, P.J., and D.B. Dotson. 1989. Land-use history and the occurrence of exotic earthworms
in the mountains of eastern Kentucky (USA). American Midland Naturalist
122:288–297.
Kalisz, P.J., and H.B. Wood. 1995. Native and exotic earthworms in wildland ecosystems.
Pp. 117–126, In P. Hendrix (Ed.). Earthworm Ecology and Biogeography in North
America. Lewis Publishers, Boca Raton, FL. 256 pp.
Kuchler, A.W. 1964. Potential natural vegetation of the conterminous United States. Special
Publication No. 36. American Geographic Society, New York, NY. 77 pp.
Lachnicht, S.L., P.F. Hendrix, and X. Zou. 2002. Interactive effects of native and exotic
earthworms on resource use and nutrient mineralization in a tropical wet forest soil of
Puerto Rico. Biology and Fertility of Soil 36:43–52.
Langmaid, K.K. 1964. Some effects of earthworm invasion in virgin podsols. Canadian
Journal of Soil Science 44:34–37.
Lawrence, A.P., and M.A. Bowers. 2002. A test of the “hot” mustard extraction method of
sampling earthworms. Soil Biology and Biochemistry 34:549–552.
Lawrence, B., M.C. Fisk, T.J. Fahey, and E.R. Suárez. 2003. Influence of nonnative earthworms
on mycorrhizal colonization of Sugar Maple (Acer saccharum). New Phytologist
157:145–153.
Li, X., M.C. Fisk, T.J. Fahey, and P.J. Bohlen. 2003. Influence of earthworm invasion on
soil microbial biomass and activity in a northern hardwood forest. Soil Biology and
Biochemistry 34:1929–1937.
Lubbers, I.M., K.J. van Groenigen, S.J. Fonte, J. Six, L. Brussaard, and J.W. van Groenigen.
2013. Greenhouse-gas emissions from soils increased by earthworms. Nature Climate
Change 3:187–194.
Luken, J.O. 1988. Population structure and biomass allocation of the naturalized shrub
Lonicera maackii (Rupr.) Maxim. in forest and open habitats. American Midland Naturalist
119:258–267.
McLean, M.A., and D. Parkinson. 1997a. Changes in structure, organic matter, and microbial
activity in pine-forest soil following the introduction of Dendrobaena octaedra
(Oligochaeta, Lumbricidae). Soil Biology and Biochemistry 29:537–540.
McLean, M.A., and D. Parkinson. 1997b. Soil impacts of the epigeic earthworm Dendrobaena
octaedra on organic matter and microbial activity in Lodgepole Pine forest. Canadian
Journal of Forest Research 27:1907–1913.
McLean, M.A., and D. Parkinson. 1998. Impacts of the epigeic earthworm Dendrobaena
octaedraon microfungal community structure in pine forest floor: A mesocosm study.
Applied Soil Ecology 8:61–75.
McLean, M.A., and D. Parkinson. 2000a. Impacts of the epigeic earthworm Dendrobaena
octaedra on microfungal community structure in pine forest floor: A mesocosm study.
Applied Soil Ecology 8:61–75.
McLean, M.A., and D. Parkinson. 2000b. Field evidence of the effects of the epigeic earthworm
Dendrobaena octaedra on the microfungal community in pine-forest floor. Soil
Biology and Biochemistry 32:351–360.
Southeastern Naturalist
83
K.N. Hopfensperger and S. Hamilton
2015 Vol. 14, No. 1
National Oceanic and Atmospheric Administration (NOAA). 2013. National Weather Service
Climate Data Graph for Covington, KY. Available online at http://www.nws.noaa.
gov. Accessed 26 September 2013.
Nelson, D.W., and L.E. Sommers. 1996. Total carbon, organic carbon, and organic matter.
Pp. 961–1010, In D.L. Sparks (Ed). Methods of Soil Analysis. Part 3, Chemical Methods.
Soil Science Society of America, Madison, WI. 1264 pp.
Nuzzo, V.A., J.C. Maerz, and B. Blossey. 2009. Earthworm invasion as the driving force
behind plant invasion and community change in northeastern North American forests.
Conservation Biology 23:966–974.
Olson, H.W. 1928. The Earthworms of Ohio. Ohio Biological Survey Bulletin 17(4):1–52.
Patton, C.J., and J.R. Kryskalla. 2011. Colorimetric determination of nitrate plus nitrite
in water by enzymatic reduction, automated discrete analyzer methods. US Geological
Survey Techniques and Methods, Book 5, Chapter B8. Reston, VA. 34 pp.
Prati, D., and O. Bossdorf. 2004. Allelopathic inhibition of germination by Allaria petiolata
(Brassicaceae). American Journal of Botany 91:285–288.
Ray, L.L. 1974. Geomorphology and quaternary geology of the glaciated Ohio River Valley:
A reconnaissance. US Department of the Interior, Geological Survey Professional,
Washington, DC. 826 pp.
Regnier, E., S.K. Harrison, J. Liu, J.T. Schmoll, C.A. Edwards, N. Arancon, and C. Holloman.
2008. Impact of an exotic earthworm on seed dispersal of an indigenous US weed.
Journal of Applied Ecology 45:1621–1629.
Reynolds, J.W. 1970. The relationship of earthworm (Oligochaeta: Lumbricidae and Megasco-
lecidae) distribution and biomass to soil type in forest and grassland habitats at Oak
Ridge National Laboratory. The Association of Southeastern Biologists Bulletin 17:60.
Reynolds, J.W. 1972. Earthworms (Lumbricidae) of the Haliburton Highlands, Ontario,
Canada. Megadrilogica 1:2–11.
Reynolds, J.W., E.E.C. Clebsch, and W.M Reynolds. 1974. Contributions to North American
earthworms: The earthworms of Tennessee (Oligochaeta). I. Lumbricidae. Bulletin
of Tall Timbers Research Station 17:1–133.
Reynolds, J.W., D.R. Linden, and C.M. Hale. 2002. The earthworms of Minnesota (Oligochaeta:
Acanthodrilidae, Lumbricidae, and Megascolecidae). Megadrilogica 8:85–98.
Rhine, E.D., G.K. Sims, R.L. Mulvaney, and E.J. Pratt. 1998. Improving the Berthelot
reaction for determining ammonium in soil extracts and water. Soil Science Society of
America Journal 62:473–480.
Ringuet, S., L. Sassano, and Z.I. Johnson. 2011. A suite of microplate reader-based colorimetric
methods to quantify ammonium, nitrate, orthophosphate, and silicate concentrations
for aquatic nutrient monitoring. Journal of Environmental Monitoring 13:370–376.
Robertson, G.P, P. Sollins, B.G. Ellis, and K. Lajtha. 1999. Exchangeable ions, pH, and
cation exchange capacity. Pp. 106–114, In G.P. Robertson, D.C. Coleman, C.S. Bledsoe,
and P. Sollins (Eds.). Standard Soil Methods for Long-term Ecological Research. Oxford
University Press, New York, NY. 462 pp.
Sackett, T.E., S.M. Smith, and N. Basiliko. 2012. Indirect and direct effects of exotic
earthworms on soil nutrient and carbon pools in North American temperate forests. Soil
Biology and Biochemistry 57:459–467.
SAS Institute, Inc., 1985. SAS User’s Guide: Statistics, Version 5. SAS Institute, Inc,
Cary, NC.
Shannon, C.E., and W. Weaver. 1949. The Mathematical Theory of Communication. University
of Illinois Press, Urbana, IL. 144 pp.
Sims, G.K. 2006. Letter to the Editor on ‘‘Using the Berthelot method for nitrite and nitrate
analysis.’’ Soil Science Society of America Journal 70:1038.
Southeastern Naturalist
K.N. Hopfensperger and S. Hamilton
2015 Vol. 14, No. 1
84
Sims, G.K., T.R. Ellsworth, and R.L. Mulvaney. 1995. Microscale determination of inorganic
nitrogen in soil and water extracts. Communications in Soil Science and Plant
Analysis 26:303–316.
Stebbings, J.H. 1962. Endemic–exotic earthworm competition in the American Midwest.
Nature 196:905–906.
Stoscheck, L.M., R.E. Sherman, E.R. Suárez, and T.J. Fahey. 2012. Exotic earthworm distributions
did not expand over a decade in a hardwood forest in New York state. Applied
Soil Ecology 62:124–130.
Suárez, E., T.J. Fahey, P.M. Groffman, P.J. Bohlen, and M.C. Fisk. 2004. Effects of exotic
earthworms on soil phosphorus cycling in two broadleaf temperate forests. Ecosystems
7:28–44.
Suárez, E., T.J. Fahey, J.B. Yavitt, P.M. Groffman, and P.J. Bohlen. 2006a. Patterns of litter
disappearance in a northern hardwood forest invaded by exotic earthworms. Ecological
Applications 16:154–165.
Suárez, E., G.L. Tierney, T.J. Fahey, and R. Fahey. 2006b. Exploring patterns of exotic
earthworm distribution in a temperate hardwood forest in south-central New York, USA.
Landscape Ecology 21:297–306.
Subler, S., C.M. Baranski, and C.A. Edwards. 1997. Earthworm additions increased shortterm
nitrogen availability and leaching in two grain-crop agroecosystems. Soil Biology
and Biochemistry 29:413–21.
Szafoni, R.E. 1991. Vegetation management guideline: Multiflora Rose (Rosa multiflora
Thunb.). Natural Areas Journal 11:215–216.
Terhivuo, J., and A. Saura. 2006. Dispersal and clonal diversity of North-European parthenogenetic
earthworms. Biological Invasions 8:1205–1218.
Tiunov, A.V., C.M. Hale, A.R. Holdsworth, and T.S. Perel. 2006. Invasion patterns of
Lumbricidae into the previously earthworm-free areas of northeastern Europe and the
western Great Lakes Region of North America. Biological Invasions 8:1223–1234.
Wardle, D. 2002. Communities and Ecosystems: Linking the Aboveground and Belowground
Components. Princeton University Press, Princeton, NJ. 408 pp.
Winsome, T., L. Epstein, P.F. Hendrix, and W.R. Horwath. 2006. Habitat quality and interspecific
competition between native and exotic earthworm species in a California
grassland. Applied Soil Ecology 32:38–53.